My immediate career goal is to become an independent investigator of stem cells, focusing on hematopoietic stem cell differentiation at the single cell level under clinically relevant stresses. My long-term research goal is to determine how hematopoietic stem cells are coordinated in sustaining a balanced blood system and how the regulatory mechanisms of this coordination are related to blood disorders. I have a strong background in molecular biology from my PhD training at Princeton University under the guidance of Dr. Ihor R. Lemischka and expertise in cell biology under the mentoring of Dr. Irving L. Weissman at Stanford University. I am proficient with several programming languages including C/C++, R, Matlab, and Python, and I can independently design and carry out the advanced statistical analyses required for modern quantitative biology. Using these cross-disciplinary skills, I have recently developed a novel, single cell in vivo tracking system featurig high sensitivity and high throughput. Our preliminary studies using this system confirm previous reports of HSC lineage bias in mice. In addition, we have discovered a previously undetected phenomenon: the majority of blood cells after irradiation-mediated transplantation are derived from the dramatic expansion of a small subset of engrafted HSC clones. More strikingly, while lineage bias and clonal expansion are commonly observed after irradiation-mediated transplantation, they are not present after unconditioned transplantation. Therefore, we hypothesize that lineage bias and clonal expansion are not innate HSC characteristics, but instead are behaviors initiated by exogenous hematopoietic stresses such as irradiation. In this grant, we propose to further test this hypothesis in mice under various transplantation conditions (K99 phase) as well as to determine whether lineage bias and clonal expansion are related (K99 phase / R00 phase) and whether they are innate deterministic features of HSC clones (R00 phase). In addition to these in vivo mice studies, we will also investigate human HSCs xenotransplanted into mice at the clonal level (R00 phase). These proposed studies will elucidate how hematopoietic homeostasis is re-established after disruption and will separate irradiation-induced effects from natural innate cellular properties. The results will help to improe the treatment of diseases associated with an unbalanced blood system and may lead to solutions that reduce the side effects of radiation in clinical applications and in occupations wit constant low levels of radiation exposure. My K99 phase training will be mentored by the distinguished hematopoietic stem cell expert Dr. Irving L. Weissman, in a world-class institutional environment at the Institute for Stem Cell Biology and Regenerative Medicine at Stanford University School of Medicine. In addition to allowing me to finish the proposed research, the K99 training will also substantially enhance my knowledge and experience with the clinical applications of my research. In addition, it will help me locate an independent research position and provide the initial support to prepare for my first R01 application.